1. Field of the Invention
The present invention relates to a method for irradiating a laser light and a laser irradiation apparatus (an apparatus including a laser and an optical system for leading laser light emitted from the laser to a body to be irradiated) for carrying out thereof. In addition, the invention relates to a semiconductor device manufactured by including an annealing (hereinafter, referred to as a laser annealing) a semiconductor film using the laser into a step and to a method for manufacturing thereof.
2. Description of the Related Art
In recent years, a technique to crystallize and to improve crystallinity by carrying out a laser annealing on a semiconductor film formed over an insulating substrate such as a glass has been studied widely. A material having silicon is used often for the semiconductor film. In this specification, a means for obtaining a crystalline semiconductor film by crystallizing a semiconductor film with a laser light is referred to as a laser crystallization. Note that, in this specification, a crystalline semiconductor film is referred to as a semiconductor film wherein a crystallized region exists, which includes a semiconductor film wherein the entire surface is crystallized.
Compared with the synthesize quartz glass substrate which is conventionally used often, a glass substrate is cheap and rich in workability, and has an advantage that a large area substrate can be easily manufactured. This is the reason for carrying out the above research. In addition, the reason why laser is used willingly for a crystallization is because a melting point of a glass substrate is low. A laser can give high energy only to a semiconductor film without raising temperature of a substrate too much. Furthermore, a throughput is extremely high in comparison with a means for heating using an electrically heated reactor.
A crystalline semiconductor film formed by carrying out a laser annealing has high mobility. Therefore, a thin film transistor (TFT) is formed using the crystalline semiconductor film, which is actively used for, for example, a monolithic type liquid crystal electro-optic device wherein TFTs for a pixel drive and a driver circuit are manufactured over a glass substrate.
An example of a method for laser annealing is a method for carrying out an annealing by moving an irradiated position of a laser beam relatively to a body to be irradiated, such that a laser beam of pulsed oscillation, typified by an excimer laser is shaped with an optical system for the purpose of being a square spot with a side of several cm or being a linear shape of 100 mm or more in length. Note that, “linear shape” here does not mean “line” in the proper sense, but means a rectangle (or, oblong) with a high aspect ratio. For example, an aspect ratio of 2 or more (preferably, 10 to 10000) is denoted; however, it may be considered that a shape on an irradiated surface is included in a laser beam (rectangular-shaped beam) that is a rectangular shape. Note that, the laser beam is shaped in a linear shape to ensure an energy density for carrying out enough annealing on a body to be irradiated, and it does not matter whether it is a rectangular shape or a planar shape as long as to carry out enough of an annealing on a body to be irradiated.
In particular, productivity is high when the linear beam is employed since an entire surface to be irradiated can be irradiated with a laser by scanning only to the direction perpendicular to the direction of the major axis of the linear beam, which is different from the case that laser light in a spot shape that a scanning is necessary in back and forth and around is used. The linear beam is scanned to the direction perpendicular to the direction of the major axis since it is a most effective scanning direction. According to this high productivity, it is becoming a mainstream for a manufacturing technique of a liquid crystal display device using a TFT to use a linear beam obtained by processing pulsed excimer laser light with a suitable optical system under a present method of laser annealing (for example, patent document 1 is to be referred to).
[Patent Document 1] Japanese Patent Application Laid-Open No. 8-195357
However, crystallization by irradiation of a laser light forms a plurality of convex portions (ridges) on the surface of a crystalline semiconductor film that is obtained, which decreases a film quality. In other words, when the laser light is irradiated on a semiconductor film, the semiconductor film is momentarily molten and locally expanded, which forms a ridge is formed on the surface of a crystalline semiconductor film in order to absorb internal stress that is generated by the expansion. In addition, the greatest vertical interval between a mountain and a valley of this ridge is around 0.5 to 2 times of a film thickness.
In a TFT of insulated gate type, a potential barrier and a trap state due to a dangling bond, a distortion of a grid, and the like are formed on the ridge of the surface of the crystalline semiconductor film; therefore, an interface level between an active layer and a gate insulating film gets higher. Furthermore, an electric field is easily concentrated in a summit part of the ridge because it is steep, which will be the source of leakage current and finally generates dielectric breakdown resulting in a short-circuits. In addition, the ridge on the surface of the crystalline semiconductor film loses deposition of a gate insulating film deposited by sputtering or CVD, which decreases a reliability such as poor insulation. In recent years, miniaturization and high integration of a semiconductor device are promoted towards high efficiency and low power consumption thereof. Therefore, a technique to make a thin film of a gate insulating film with high reliability is essential; however, the ridge on the surface in a semiconductor film has a high influence on reliability of a gate insulating film, accompanying with a conversion into a thin film of a gate insulating film. In addition, a scattering effect of a surface is given as one of elements that determine an electron field-effect mobility of a TFT. Surface smoothness of interface between an active layer and of a gate insulating film of a TFT has high influence on electron field-effect mobility. Therefore, the more the interface is flat, the more high electron field-effect mobility can be obtained without being affected by scattering. Thus, the ridge affects all characteristics of a TFT on the surface of a crystalline semiconductor film, which also changes a yield rate.